The present invention relates to an internally cooled high-energy cable consisting of a plurality of consecutive sections, and more particularly to a water-cooled high-voltage high-energy cable having a closed internal cooling channel.
There are already known high-energy cables including a plurality of radially superimposed layers of various properties, which surround one another and the innermost one of which surrounds an internal cooling channel. It is already known, for transmitting energies in the order of 2,000 MVA, to provide a cable which is formed with an internal cooling channel bounded by the electric conductor itself. Experience with this type of cable has shown that, when the electric conductor is made of aluminum, the diameter of the cooling channel should be greater than 60 millimeters, particularly equal to or exceeding 70 millimeters.
It has also been already proposed to use water as the cooling medium for internally cooling the high-energy cable. However, may problems are encountered when water is used as the cooling medium. So, for instance, when the cooling channel is bounded by the electric conductor itself as mentioned above, that is, when the innermost layer of the cable is of a material having high electric conductivity, such as aluminum, there exists the danger that the electric conductor will be attacked by the cooling medium, that is cooling water, and will corrode over a period of time until the cable is rendered useless.
Further difficulties are encountered when the electric conductor is constituted by a plurality of layers some of which are circumferentially complete tubular electric conductors and some of which are constituted by layers or segments of elongated electrically conductive elements which surround or are surrounded by the tubular electric conductors. Such a multi-layer construction of the electric conductor is often necessary particularly where, as in the present case, the thickness of the electric conductor in the radial direction of the cable is substantial so as to permit bending of the electric conductor during the manufacture, transportation and laying of the cable. The difficulties arising from such a construction are particularly pronounced when the elongated electrically conductive elements are of the same material as the tubular electric conductor. To understand these difficulties, it is to be mentioned that only a certain length of the cable can be transported to the point of use so that the cable is usually assembled from a plurality of such lengths or sections in situ by welding the end portions of such sections to one another. During the welding operation, the consecutive inner tubular elements are welded to one another first, with the elongated electrically conductive elements removed from the region of welding, and then another welding operation is performed for connecting the elongated electrically conductive elements of the consecutive lengths of the cable to one another to form the superimposed layer of the electric conductor which surrounds the inner tubular electric conductor. It will be appreciated that, during the second welding operation, the previously manufactured welded connection of the two consecutive tubular electric conductors will be reheated to the welding temperature, that is to a temperature which at least plasticizes the material of the welded connection of the tubular electric conductors. As a result of this reheating of the welded connection, the quality thereof in most instances suffers so that it is impossible or at least very difficult to assure a faultless water-tight connection between the two inner tubular electric conductors.
Another problem to be considered when manufacturing such cables is that the materials of the various layers of the electric conductor may electrically interact with one another, particularly when having different electric potentials. In other words, such materials will act as an electric cell and thus cause corrosion of at least one of such materials. Therefore, caution must be exercised in selecting the proper materials for the various components of the cable and other members which connect the cable into an electric circuit, which come into contact with the cooling medium.
In the electric cables which have been discussed so far, there also exists the danger that, due to the relatively low resistance to wear of the electric conductor bounding the cooling channel through which the cooling water or similar cooling fluid flows at relatively high speeds, the erosion of the internal surface of the electric conductor will be relatively high, which will render the electric cable of the prior art useless within a relatively short period of time, especially after the cooling fluid starts leaking through the electric conductor. To avoid the above-discussed disadvantages, it has already been proposed to provide an inner tubular member of alloyed stainless steel or of titanium within the electric conductor proper, which inner tubular member then bounds the cooling channel for the cooling medium and prevents such cooling medium from coming into contact with the electric conductor. Then, the inner tubular members of any two consecutive sections of the cable are welded to one another first, after possibly bending the other conductor away from the connecting zone, and then the electric conductors of the consecutive cable sections are welded to one another, possibly after reinstating the previously bent portions of the electric conductors into their original positions.
Advantageous as this procedure may be in many applications, in some instances problems may develop during such connecting procedures, especially for some materials of the various components of the cable and for some connecting procedures. Also, the welding operations have to be performed with a high degree of care and skill, thus putting higher demands on the welding equipment and the operating personnel. These problems are further aggravated when the two consecutive sections are to be connected outside the manufacturing plant, that is, during the laying of the cable, as is usually the case.